Declaring a Nullable String

Hello,
How do I allow nullable string to my declaration?
I get this error "Type 'String' must be a value type or a type argument constrained to 'Structure' in order to be used with 'Nullable' or nullable modifier '?'."

Function Index(ByVal product As Product, ByVal category_1 As Nullable(Of String), ByVal category_2 As Nullable(Of String)) As ActionResult
...
End Function


Thank you for your help guys!


-imperialx



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